The present invention is related to water heating systems and more specifically to heating systems for ultrapure water.
Some purification systems produce ultrapure water. Such systems remove from a water supply particulates, ions, organic matter and microbes that could otherwise contaminate the process or device that uses the ultrapure water. One example of a system for producing ultrapure water is described in Sauer and Vedova, xe2x80x9cA New Water Treatment System for the latest Generation of Semiconductor Devicesxe2x80x9d, Ultrapure Water, December 1996.
Some processes require heated ultrapure water or ultrapure steam. For example, some manufacturing processes may use heated ultrapure water for producing and cleaning semiconductor wafers used to manufacture electronic devices. Other manufacturing processes may use heated ultrapure water for producing and cleaning components used to manufacture disk drives. Pharmaceutical and biotechnology manufacturing processes use heated ultrapure water for cleaning and sterilizing, and can also include ultrapure water in the finished product. In addition, heated ultrapure water may be used to clean the filters that make up the purification system. It is therefore desirable to heat ultrapure water.
Many conventional techniques exist for heating water. For example, conventional systems for heating water may use heat exchange techniques. In these techniques, heat is transferred from a liquid or gas to the liquid to be heated. Conventional heat exchange techniques use a plate and frame, double pipe, shell and tube, or other form of heat exchanger to transfer heat from a non-purified aqueous- or steam- heat source to the liquid to be heated, separated by a heat conductor. Other heat exchange techniques include cross flow systems in which heated air is passed over pipes containing the water. The pipes used in the heat exchanger have fins to improve the effective heat transfer from the air to the water. Pipes or heat conductors for such systems may be made of aluminum, copper, stainless steel, or nickel alloys, exotic metals, such as titanium, or plastics in order to maximize the heat transfer to the water.
While such systems can provide efficient heat transfer, they can contaminate ultrapure water. When heated, the conductive materials used for the piping or heat conductors can leach particulate and ionic contamination into the ultrapure water. Titanium pipes or conductors have been used in the heat exchangers instead of the other types of pipes, but as standards for ultrapure water improve, titanium introduces unacceptable amounts of impurities into the ultrapure water. Fluoropolymer pipes have also been used in heat exchangers, but such pipes are not good conductors of heat, and thus, they adversely impact the efficiency of the heat exchange.
Other heating techniques have been attempted to heat ultrapure water such as running a thin stream of ultrapure water past a current-carrying wire used as a heat source. Here too, the contamination introduced by the heated metal wire is sufficiently high to contaminate the ultrapure water. Radiant heat can be passed through quartz pipes, but quartz pipes are fragile and relatively difficult to seal.
What is needed is a method and apparatus for heating ultrapure water while minimizing the amount of contamination introduced to the water by the heating process.
A method and apparatus uses microwave energy to heat ultrapure water in a chamber that does not release contaminants to the water when heated. Because fluid-fluid heat exchange techniques are not used to heat the water, thermally conductive materials need not be used to transport the ultrapure water, and tubing made of inert materials may be used instead, maintaining the purity of the water. Because the heat source itself is not in contact with the water, the purity of the water is maintained.